If you use only \cite in the caption and not other citations commands and no optional arguments, an alternative is to use the etoolbox to add a pre and post hook to \listoffigures and to redefine \cite

You need to change the setup of your custom command to take one additional argument, viz., the material to be written to the List of Figures. In the code below, the content of #4 is what will show up in the List of Figures; it should be quite similar to #3 except for the citation command.

\munepsfig[scale=0.5,angle=90]{barchart}{Population over time
\cite{some-source}}{Population over time}

A downside of this approach is this: if the material to be shown in the List of Figures is identical to what's to be shown in the caption, you still need to provide the fourth argument. If you don't, LaTeX will keep looking ahead in the input stream for the next token -- which could literally be anything -- and treat that as the command's fourth argument. Expect weird error messages and odd crashes if you do this...